Abstract:

Wire products, such as round and flat wire, strands, cables, and tubing,
are made from a shape memory material in which inherent defects within
the material are isolated from the bulk material phase of the material
within one or more stabilized material phases, such that the wire product
demonstrates improved fatigue resistance. In one application, a method of
mechanical conditioning in accordance with the present disclosure
isolates inherent defects in nickel-titanium or NiTi materials in fields
of a secondary material phase that are resistant to crack initiation
and/or propagation, such as a martensite phase, while the remainder of
the surrounding defect-free material remains in a primary or parent
material phase, such as an austenite phase, whereby the overall
superelastic nature of the material is preserved.

Claims:

1. A medical device comprising a wire made of a nickel-titanium shape
memory material, said wire having a fatigue endurance exceeding 0.95%
strain amplitude at greater than 10.sup.6 cycles.

2. The medical device of claim 1, wherein said wire has a fatigue
endurance exceeding 1.1% strain amplitude at greater than 10.sup.6
cycles.

3. The medical device of claim 1, wherein said wire has a fatigue
endurance exceeding 1.1% strain amplitude at greater than 10.sup.9
cycles.

4. The medical device of claim 1, wherein said wire has a residual strain
of less than 0.25% after being subjected to engineering strain of at
least 9.5%.

5. A medical device comprising a wire product made of a shape memory
material, said shape memory material having a plurality of defects, said
wire product substantially comprised of said shape memory material in a
primary phase and including portions of said shape memory material
comprising a secondary phase at localized regions disposed proximate
respective said defects, with at least some of said secondary phase
portions separated by said primary phase.

6. The medical device of claim 5, wherein said shape memory material is a
nickel-titanium shape memory material, said primary phase is an austenite
phase, and said secondary phase portions comprise a martensite phase.

7. The medical device of claim 5, wherein said secondary phase portions
together comprise less than 15% of the shape memory material, by volume.

8. The medical device of claim 5, wherein said shape memory material is a
nickel-titanium shape memory material, and said wire product has a
fatigue endurance exceeding 0.95% strain amplitude at greater than
10.sup.6 cycles.

9. The medical device of claim 5, wherein said shape memory material is a
nickel-titanium shape memory material, and said wire product has a
fatigue endurance exceeding 1.1% strain amplitude at greater than
10.sup.6 cycles.

10. The medical device of claim 5, wherein said shape memory material is a
nickel-titanium shape memory material, and said wire product has a
fatigue endurance exceeding 1.1% strain amplitude at greater than
10.sup.9 cycles.

11. The medical device of claim 5, wherein said wire has a residual strain
of less than 0.25% after being subjected to engineering strain of at
least 9.5%.

12. The medical device of claim 5, wherein said wire product is selected
from the group consisting of wire having a circular cross-section, wire
having a non-circular cross-section, cable, coil, and tubing.

13. A method, comprising the steps of:providing a wire product made of a
shape-set, shape memory material;mechanically conditioning the wire
product by:applying an engineering stress between 700 MPa and 1600 MPa;
andreleasing the applied engineering stress; andincorporating the wire
product into a medical device.

21. The method of claim 19, wherein said mechanical conditioning step
further comprises:applying the first force to the wire product in an
environment having a temperature T, whereinT=Af.+-.50.degree.
C.,wherein Af is the austenite transformation finish temperature of
the nickel-titanium shape memory material.

22. The method of claim 13, wherein the wire product is selected from the
group consisting of wire having a circular cross-section, wire having a
non-circular cross-section, cable, coil, and tubing.

[0003]The present disclosure relates to fatigue damage resistant wire and,
in particular, relates to a method of manufacturing wire made of a shape
memory alloy, which demonstrates improved fatigue strength properties, as
well as medical devices made with such wire.

[0004]2. Description of the Related Art

[0005]Shape memory materials are materials that "remember" their original
shape, and which, after being deformed, return to that shape either
spontaneously or by applying heat to raise their temperature above a
processing and material related threshold known as the transformation
temperature. Heating to recover shape is commonly referred to in the art
as "shape memory", whereas spontaneous recovery is commonly referred to
as pseudoelasticity. Pseudoelasticity, sometimes called superelasticity,
is a reversible response to an applied stress, caused by a phase
transformation between the austenite or parent phase and the martensite
or daughter phase of a crystal. It is exhibited in shape memory alloys.
Pseudoelasticity and shape memory both arise from the reversible motion
of domain boundaries during the phase transformation, rather than just
bond stretching or the introduction of defects in the crystal lattice. A
pseudoelastic material may return to its previous shape after the removal
of even relatively high applied strains by heating. For example, even if
the secondary or daughter domain boundaries do become pinned, for example
due to dislocations associated with plasticity, they may be reverted to
the primary or parent phase by stresses generated through heating.
Examples of shape memory materials include iron-chrome-nickel,
iron-manganese, iron-palladium, iron-platinum,
iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron,
copper-zinc-aluminum, copper-zinc-aluminum-nickel,
copper-aluminum-nickel, and nickel-titanium alloys. Shape memory
materials can also be alloyed with other materials including zinc,
copper, gold, and iron.

[0006]Shape memory materials are presently used in a variety of
applications. For example, a variety of military, medical, safety and
robotic applications for shape memory materials are known. Medical grade
shape memory materials are used for orthodontic wires, guide wires to
guide catheters through blood vessels, surgical anchoring devices and
stent applications, for example. One shape memory material in wide use,
particularly in medical device applications, is a nickel-titanium shape
memory material known as "Nitinol".

[0007]Many medical grade shape memory wire products are made of
biocompatible implant grade materials including "NiTi" materials. As used
herein, "nickel-titanium material", "nickel-titanium shape memory
material" and "NiTi" refer to the family of nickel-titanium shape memory
materials including Nitinol (an approximately equiatomic nickel-titanium,
binary shape memory material) as well as alloys including nickel and
titanium as primary constituents but which also include one or more
additional elements as secondary constituents, such as Nitinol tertiary
or quaternary alloys (Nitinol with additive metals such as chromium,
tantalum, palladium, platinum, iron, cobalt, tungsten, iridium and gold).

[0008]Significant research has been dedicated to understanding how NiTi
behaves in the body from the viewpoint of biological host response, but
much less has been published that quantitatively correlates structure
with mechanical properties.

[0009]More particularly, the fatigue properties of NiTi material have been
the subject of recent research. The fatigue crack propagation behavior of
Nitinol was studied in detail by McKelvey and Ritchie, as published in
Fatigue-Crack Growth Behavior in the Superelastic and Shape-Memory Alloy
Nitinol, Metallurgical and Materials Transactions, 32A, 2001, pgs.
731-743. McKelvey et al. observed that the crack growth propagation rate
and ΔKth, which denotes the stress-intensity fatigue threshold
in a given fatigue-crack growth scenario, were different for equivalent
composition at martensite-stable and austenite-stable temperatures where
the crack growth rate was generally lower at martensite-stable
temperatures. They also observed that, under plane strain conditions, the
heavily slipped material near the crack tip at superelastic regime
temperatures remained austenitic, presumably inhibited from undergoing
volume contractile, stress-induced phase transformation by the triaxial
stress state, while plane stress conditions generally resulted in
stress-induced martensite near the crack tip.

[0010]Wire products made of shape memory materials are manufactured by
forming a relatively thick piece of hot-worked rod stock from a melt
process. The rod stock is then further processed into wires by drawing
the rod stock down to a thin diameter wire. During a drawing process,
often referred to as a "cold working" process, a wire is pulled through a
lubricated die to reduce its diameter. The deformation associated with
wire drawing increases the stress in the material, and the stress
eventually must be relieved by various methods of heat treatment or
annealing at elevated temperatures to restore ductility, thus enabling
the material to be further cold worked to a smaller diameter.
Conventional wire annealing typically results in grain growth with a
concomitant random crystal orientation, and the various material or fiber
"textures" that are generated during cold wire drawing are mostly
eliminated during conventional annealing and recrystallization. These
iterative processes of cold working and annealing may be repeated several
times before a wire of a desired diameter is produced and processing is
completed.

[0011]Wire materials manufactured by the above processes typically contain
microstructural defects, such as pores, inclusions, interstitials, and
dislocations. An inclusion comprises a phase which possesses distinct
properties from the primary material matrix and is divided from the
matrix by a phase boundary. Inclusions may result from oxide or other
metallic or non-metallic precipitate formation during primary melting or
other high temperature treatment and may include carbides, nitrides,
silicides, oxides or other types of particles. Inclusions may also arise
from contamination of the primary melt materials or from the mold which
contains the molten ingot. In the case of an interstitial, an atom
occupies a site in the crystal structure at which there usually is not an
atom. The atom may be a part of its host material, such as a base metal
or alloying metal, or it may be an impurity. A dislocation is a linear
defect around which some of the atoms of the crystal lattice are
misaligned and appear as either edge dislocations or screw dislocations.
Edge dislocations are caused by the termination of a plane of atoms in
the middle of a crystal, while a screw dislocation comprises an internal
structure in which a helical path is traced around the linear defect or
dislocation line by the atomic planes of atoms in the crystal lattice.
Mixed dislocations, combining aspects of screw and edge dislocations, may
also occur.

[0012]Internal or external defects, such as inclusions, pores, or defects
induced during wire processing may weaken the host material at the site
of the defect, potentially resulting in failure of a material at the site
of that defect. This weakening may be particularly acute where the defect
is relatively large and/or of significantly disparate stiffness compared
with adjacent dimensions of the material (such as for fine or small
diameter wire). Failure of shape memory wires is more likely to occur at
the site of the defect. Since inherent defects cannot be completely
eliminated from the wire material, management of inherent defects and
mitigation of their negative impact on wire properties is desirable.

[0013]One previously proposed solution to the problem of inherent defects
has been to treat selected regions of a wire that are expected to be
subjected to high strain by converting the bulk material in such regions
to a different phase than the remainder of the bulk material of the wire.
For example, under predetermined operating conditions, such as a
predetermined operation temperature, the high strain wire regions are
stabilized in a martensite phase while the lesser strain regions remain
in an austenite phase. This method is therefore directed to treating
predetermined regions of a wire to convert the bulk material in the
regions to a more stable phase regardless of the presence, number, and
location of any defects in the bulk material.

[0014]However, it may not always be possible or practical to predict what
regions of a continuous wire will be subjected to high strains when
portions of the wire are later incorporated into a medical device. It may
also be desirable to leave defect-free portions of wire unaffected by
mitigation efforts and, therefore, available to meet other design
considerations. For example, a disadvantage of the above process is that
for wire made of shape memory material, the regions that are stabilized
in the martensite phase will lose the superelastic characteristic.

[0015]Although wires made in accordance with foregoing processes may
demonstrate excellent fatigue strength, further improvements in fatigue
strength are desired, particularly with reference to fatigue damage that
propagates from defects.

[0016]What is needed is a method of manufacturing a wire that demonstrates
improved fatigue strength, and medical devices that include such wire.

SUMMARY

[0017]The present disclosure relates to wire products, and medical devices
including wire products, such as round and flat wire, strands, cables,
coils, and tubing, made from a shape memory material or alloy. Defects
within the material are isolated from a primary, or parent, material
phase within one or more areas of stabilized secondary, or daughter,
material phases that are resistant to failure, such that the wire product
demonstrates improved fatigue strength. In one application, a method of
mechanical conditioning in accordance with the present disclosure
isolates defects in nickel-titanium or NiTi shape memory materials in
localized areas or fields of a secondary material phase that are
resistant to crack initiation and/or propagation, such as a martensite
phase, while the remainder of the surrounding defect-free material
remains in a primary material phase, such as an austenite phase, whereby
the overall superelastic and/or nature of the material is preserved.

[0018]Wire products manufactured in accordance with the present disclosure
maintain good mechanical properties in addition to improved fatigue
performance Increases in the strain fatigue limit for both high cycle and
low cycle fatigue are observed, while shape memory or superelastic
characteristics are preserved.

[0019]As discussed below and shown in the Working Examples, the amount of
secondary phase material formed about the defects during the mechanical
conditioning process is sufficient to either completely isolate the
defects or at least partially isolate high stress concentrator areas
about the defects in order to the improve fatigue strength of the
material and yet, when the bulk of the material reverts back to the
primary phase after the mechanical conditioning, the overall amount of
remaining secondary phase material that is formed about the defects is
not sufficient compromise the shape memory or superelastic characteristic
of the material as a whole. In this respect, the amount of mechanical
conditioning may be specifically tailored to achieve a desired balance
between fatigue strength and material elasticity.

[0020]In one form thereof, the present invention provides a medical device
including a wire made of a nickel-titanium shape memory material, the
wire having a fatigue endurance exceeding 0.95% strain amplitude at
greater than 106 cycles.

[0021]In other embodiments, the medical device may include a wire having a
fatigue endurance exceeding 1.1% strain amplitude at greater than
106 cycles, or a fatigue endurance exceeding 1.1% strain amplitude
at greater than 109 cycles. In a further embodiment, the medical
device may include a wire having a residual strain of less than 0.25%
after being subjected to engineering strain of at least 9.5%.

[0022]In another form thereof, the present invention provides a medical
device including a wire product made of a shape memory material, the
shape memory material having a plurality of defects, the wire product
substantially comprised of the shape memory material in a primary phase
and including portions of the shape memory material comprising a
secondary phase at localized regions disposed proximate respective
defects, with at least some of the secondary phase portions separated by
the primary phase.

[0023]The shape memory material may be a nickel-titanium shape memory
material, in which the primary phase is an austenite phase, and the
secondary phase portions comprise a martensite phase. The secondary phase
portions may together comprise less than 15% of the shape memory
material, by volume.

[0024]In a further embodiment, the shape memory material may be a
nickel-titanium shape memory material, with the wire product having a
fatigue endurance exceeding 0.95% strain amplitude at greater than
106 cycles, a fatigue endurance exceeding 1.1% strain amplitude at
greater than 106 cycles, or a fatigue endurance exceeding 1.1%
strain amplitude at greater than 109 cycles. The wire may also have
a residual strain of less than 0.25% after being subjected to engineering
strain of at least 9.5%. The wire product may be selected from the group
consisting of wire having a circular cross-section, wire having a
non-circular cross-section, cable, coil, and tubing.

[0025]In a further form thereof, the present invention provides a method,
including the steps of: providing a wire product made of a shape-set,
shape memory material; mechanically conditioning the wire product by:
applying an engineering stress between 700 MPa and 1600 MPa; and
releasing the applied engineering stress; and incorporating the wire
product into a medical device. The mechanical conditioning step may occur
either prior to or after the incorporation step.

[0026]In another embodiment, the mechanical conditioning step includes:
applying an engineering stress between 900 MPa and 1450 MPa; and
releasing the applied engineering stress. In a further embodiment, the
mechanical conditioning step includes: applying an engineering stress
between 1100 MPa and 1350 MPa; and releasing the applied engineering
stress. The method may further include the repeating the mechanically
conditioning step at least once.

[0027]In one embodiment, the shape memory material may be a
nickel-titanium shape memory material, and the mechanical conditioning
step may be conducted below a martensite deformation temperature
(Md) of the nickel-titanium shape memory material. The mechanical
conditioning step may further include: applying the first force to the
wire product in an environment having a temperature T, wherein

T=Af±50° C.,

wherein Af is the austenite transformation finish temperature of the
nickel-titanium shape memory material. The wire product may be selected
from the group consisting of wire having a circular cross-section, wire
having a non-circular cross-section, cable, coil, and tubing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]The above-mentioned and other features and advantages of this
invention, and the manner of attaining them, will become more apparent
and the invention itself will be better understood by reference to the
following descriptions of embodiments of the invention taken in
conjunction with the accompanying drawings, wherein:

[0029]FIG. 1 is a schematic view of a portion of wire having an equiaxed
grain structure;

[0030]FIG. 2 is a schematic view of the portion of wire of FIG. 1 having
an elongated grain structure after cold work conditioning;

[0031]FIG. 3 is a schematic view of the portion of wire of FIG. 2 having
an equiaxed grain structure with smaller grains than the equiaxed grain
structure of the wire in FIG. 1 after a shape set annealing process;

[0032]FIG. 4 is a schematic view illustrating an exemplary drawing process
using a lubricated die;

[0033]FIG. 5 is a depiction of the processing step of conditioning a wire
using a mechanical conditioning method in accordance with the present
disclosure;

[0034]FIG. 6 is a depiction of the processing step of releasing the
tension in the wire of FIG. 6;

[0035]FIG. 7(a) is a view of a portion of a wire having internal and
external defects;

[0036]FIG. 7(b) is an fragmentary view of a defect in the wire of FIG.
7(a);

[0037]FIG. 8 is a stress-strain curve for a mechanical conditioning
process in accordance with the present disclosure;

[0038]FIG. 9 is a view of a portion of a wire having internal and external
defects substantially surrounded by dislocation-stabilized secondary
phase;

[0039]FIG. 10(a) is a fragmentary view of a defect in the wire of FIG. 9
substantially surrounded by dislocation-stabilized secondary phase;

[0040]FIG. 10(b) is a view of a portion of a wire having internal and
external defects substantially surrounded by dislocation-stabilized
secondary phase;

[0041]FIG. 10(c) is a view of a portion of a wire having internal and
external defects substantially surrounded by dislocation-stabilized
secondary phase;

[0049]FIG. 13(b) is an enlarged insets showing a loading region of the
graph shown in FIG. 13(a);

[0050]FIG. 13(c) is an enlarged insets showing an unloading region of the
graph shown in FIG. 13(a);

[0051]FIG. 14 is a graphical representation of rotary bend fatigue data
for conditioned (C) and non-conditioned (NC) samples under the following
test conditions: T=300 K, rate=60 s-1, R=-1, with a maximum stress
error=3% and a maximum cycle count error=0.5%;

[0052]FIG. 15 is a graphical representation of single test level (1%
alternating engineering strain) data for FIB-sharp defect (FSD) and
FSD-conditioned (FSD-C) samples, with the extension bars in the inset
representing the data spread for n=3 samples;

[0053]FIG. 16 is a bright field TEM (BF-TEM) image of an FSD crack root
after mechanical conditioning, with the insets showing selected area
electron diffraction patterns (SADP) for regions within (left) and
outside of (right) the structurally distinct zone demarcated by a dashed
line and extending approximately 0.5 nm from the crack tip;

[0054]FIG. 17 is a graphical representation of crack growth rate data
inferred from high resolution scanning electron microscopy of ductile
striation spacing observations and estimated stress intensity at probable
crack front location based on a semi-elliptical crack in an infinite rod;

[0055]FIG. 18(a) is a graph showing cycles to failure for five sets of
wire samples, where a sample from each set of wires has been mechanically
conditioned with a given level of engineering stress, and where the wires
were tested at a 1.25% strain level;

[0056]FIG. 18(b) is a graph showing cycles to failure for the five sets of
wire samples shown in FIG. 18(a), where a sample from each set of wires
has been mechanically conditioned with a given level of engineering
stress, and where the wires were tested at a 1.1% strain level;

[0057]FIG. 18(c) is a graph showing cycles to failure for the five sets of
wire samples shown in FIG. 18(a), where a sample from each set of wires
has been mechanically conditioned with a given level of engineering
stress, and where the wires were tested at a 0.95% strain level;

[0058]FIG. 18(d) is a graph showing cycles to failure for the five sets of
wire samples shown in FIG. 18(a), where a sample from each set of wires
has been mechanically conditioned with a given level of engineering
stress, and where the wires were tested at a 0.80% strain level;

[0059]FIG. 19(a) is a stress-strain curve for five wire samples, where
each wire sample was loaded using the conditioning regime indicated by
the legend at the right of the figure and described in Table 2;

[0060]FIG. 19(b) is a stress-strain curve for five wire samples, where
each wire sample was loaded using the conditioning regime indicated by
the legend at the right of the figure and described in Table 2;

[0061]FIG. 19(c) is a stress-strain curve for five wire samples, where
each wire sample was loaded using the conditioning regime indicated by
the legend at the right of the figure and described in Table 2;

[0062]FIG. 19(d) is a stress-strain curve for five wire samples, where
each wire sample was loaded using the conditioning regime indicated by
the legend at the right of the figure and described in Table 2;

[0063]FIG. 19(e) is a stress-strain curve for five wire samples, where
each wire sample was loaded using the conditioning regime indicated by
the legend at the right of the figure and described in Table 2;

[0064]FIG. 20 is a graph showing the percentage of isothermally
non-recoverable strain in various wire materials as a function of a
mechanical conditioning parameter;

[0065]FIG. 21(a) is a section view of Drawn Filled Tubing (DFT®) wire
manufactured in accordance with an embodiment of the present disclosure
(DFT® is a registered trademark of Fort Wayne Metals Research
Products Corporation of Fort Wayne, Ind.);

[0066]FIG. 21(b) is a cross sectional view taken along line 18B-18B of
FIG. 18(a);

[0067]FIG. 22(a) is an elevation view of a braided tissue scaffold or
stent including a wire made in accordance with the present process; and

[0068]FIG. 22(b) is an elevation view of a knitted tissue scaffold or
stent including a wire made in accordance with the present process.

[0069]Corresponding reference characters indicate corresponding parts
throughout the several views. The exemplifications set out herein
illustrate preferred embodiments of the invention and such
exemplifications are not to be construed as limiting the scope of the
invention in any manner.

DETAILED DESCRIPTION

[0070]The present disclosure relates to wire products, and medical devices
including wire products, such as round and flat wire, strands, cables,
coils, and tubing, made from a shape memory material or alloy. Defects
within the material are isolated from a primary, or parent, material
phase within one or more stabilized secondary, or daughter, material
phases that are resistant to failure, such that the wire product
demonstrates improved fatigue strength. In one application, a method of
mechanical conditioning in accordance with the present disclosure
isolates defects in nickel-titanium or NiTi shape memory materials in
localized areas or fields of a secondary material phase that are
resistant to crack initiation and/or propagation, such as a martensite
phase, while the remainder of the surrounding defect-free material
remains in a primary material phase, such as an austenite phase, whereby
the overall superelastic nature of the material is preserved.

[0071]As used herein, a "defect" refers to material defects such as
crack-like defects, inclusions, dislocations, and other non-uniformities,
as well as any other internal or external defects or stress risers
present in a material, as well as melt intrinsic and extrinsic defects
such as inclusions, porosity, voids and oxide precipitate formation after
melting.

[0072]Exemplary manufacturing processes by which wires may be made in
accordance with the present disclosure are set forth in Section I below,
and general descriptions of the resulting physical characteristics of
wires made in accordance with the present process are set forth in
Section II below. Working Examples are set forth in Section III below.
Applications using wires made in accordance with the present disclosure
are set forth in Section IV below.

[0074]Moreover, it is contemplated that various shape-memory materials
having either a one-way memory effect or a two-way memory effect, and
other related materials, may be subjected to the present mechanical
conditioning process to achieve enhanced physical characteristics
identified in the discussion below and in the corresponding Working
Examples.

[0075]As discussed in detail in Section IV below, fatigue damage resistant
shape memory wire made in accordance with the present disclosure may be
used in medical devices such as, for example, implantable cardiac pacing,
shocking and/or sensing leads, implantable neurological stimulating
and/or sensing leads, wire-based stents, blood filter devices, or any
other medical device application in which high fatigue strength and/or a
shape memory or superelastic characteristic is desired. Wire products
produced in accordance with the present disclosure may also be used in
non-medical device applications in which high fatigue strength and/or a
shape memory or superelastic characteristic is desired.

[0076]As used herein, "wire" or "wire product" encompasses continuous wire
and wire products, such as wire having a round cross section and wire
having a non-round cross section, including flat wire, as well as other
wire-based products such as strands, cables, coil, and tubing.

I. DESCRIPTION OF THE PRESENT MANUFACTURING PROCESS

[0077]1. Wire Preparation

[0078]Prior to the mechanical conditioning process of the present
disclosure, discussed below, wire made of a shape memory material is
subjected to cold work prior to undergoing a shape set annealing process.
The shape setting step imparts the primary shape memory and/or
superelastic characteristics of the material prior to mechanical
conditioning.

[0079]Initial preparation of a wire may involve first forming a piece of
rod stock, for example, based on conventional melt processing techniques,
followed by one or more iterations of conventional cold working and
annealing. Referring to FIG. 1, a schematic or exaggerated view of a
portion of wire 10 manufactured in accordance with conventional cold
working and annealing techniques is shown. Wire 10 has been subjected to
one or more, perhaps several or a very large number of, iterations of
conventional cold working and annealing, as described above, to form an
equiaxed crystal structure within the material of wire 10. Representative
equiaxed crystals are depicted in wire 10 at 12. As used herein,
"equiaxed" refers to a crystal structure in which the individual crystals
12 have axes that are approximately the same length, such that the
crystals 12 collectively have a large number of slip planes, leading to
high strength and ductility. However, it is not necessary that the grain
structure be equiaxed. The grain structure may, for example, contain
deformed grains that have been recovered to the B2 cubic austenite phase
through the high temperature shape setting process described below.

[0080]Referring now to FIG. 2, prior to the shape-set anneal, wire 10 may
optionally subjected to further cold work in the form of a cold work
conditioning step if a nanograin microstructure is desired. As used
herein, "cold work conditioning" means imparting a relatively large
amount of cold work to a material, such as by wire drawing, swaging, or
otherwise forming.

[0081]Referring to FIG. 4, the cold work conditioning step is performed by
drawing wire 10 through a lubricated die 18 (FIG. 4) having a an output
diameter D2, which is less than diameter D1 of the undrawn wire
10 shown in FIG. 2. In one exemplary embodiment, the cold work
conditioning step by which the diameter of wire 10 is reduced from
D1 to D2 is performed in a single draw and, in another
embodiment, the cold work conditioning step by which the diameter of wire
10 is reduced from D1 to D2 is performed in multiple draws
which are performed sequentially without any annealing step therebetween.

[0082]Further discussion of exemplary cold work conditioning processes are
presented in U.S. patent application Ser. No. 12/563,062, entitled
FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed
Sep. 18, 2009, assigned to the present assignee, the disclosure of which
is hereby expressly incorporated by reference herein in its entirety. The
foregoing reference also discloses methods of limited annealing following
the cold work conditioning to create a nanograin microstructure, which
may optionally be applied to wires prior to subjecting same to the
mechanical conditioning process in accordance with the present
disclosure, discussed below.

[0083]Regardless of the amount of cold work imparted to the wire and/or
whether cold work conditioning is used, once drawn to the desired size,
and as shown in FIG. 5, wire 10 undergoes a shape setting annealing
process in which it is continuously annealed under constant tension
sufficient to hold the wire in a substantially linear configuration
during the shape set annealing process. Shape set annealing typically
occurs at a temperature between 300° C. (673 K) and 600° C.
(1073 K), where the temperature is sufficiently high to restore a
majority of the wire material to the primary or austenite phase. The
shape setting annealing process may result in the formation of a new
crystallographic structure, which may comprise nano-scale equiaxed
crystals 16 in wire 10, as shown in FIG. 3. However, as noted above, an
equiaxed or nanograin crystal microstructure is not required for the
mechanical conditioning process described below.

[0084]2. Mechanical Conditioning

[0085]In accordance with the present disclosure, wire made of a shape
memory material may be subjected to a mechanical conditioning process to
improve its resistance to fatigue damage. In the present embodiment,
mechanical conditioning is performed by application of a force to the
wire in an environment having a temperature range within approximately
50° C. of the austenite transformation finish temperature
(Af), i.e., T=Af±50° C.

[0086]Referring now to FIGS. 5 and 6, in the mechanical conditioning
process, shape memory wire, shown in FIG. 5, in the heat set annealed
condition is secured at a first end. Controlled engineering strain is
applied in the direction of arrow F to a finished engineering strain,
εc, of 0.08 to 0.14 units, wherein:

c = Δ L L o , ##EQU00001##

with Lo being the initial length, and ΔL being the length
increase imparted by strain.

[0087]The range for εc may be from as little as 0.06, 0.08,
or 0.10 units to as much as 0.13, 0.14 0.16 units, or within any range
encompassed by the foregoing values, to provide the increased benefits to
the shape memory wire discussed herein. It is thought that if the end
point is lower than 0.08, the mechanical conditioning may not impart the
desired physical properties to the wire, as any dislocation or
stress-induced secondary phase, such as martensite that is formed, may
revert back to the primary or parent phase, such as austenite. In
contrast, if the end point is in excess of 0.14, the wire may potentially
be strained by the mechanical conditioning beyond its elastic deformation
range to the extent that an undesirably large amount of plastic
deformation may result.

[0088]An alternative characterization of the mechanical conditioning
process of the present disclosure may be expressed as the application of
engineering stress to the material. For an application of force in a
tensile test as described herein, engineering stress is calculated using
the following equation:

σ e = P A 0 ##EQU00002##

where σe is the engineering stress, P is the force applied, and
A0 is the cross-sectional area of the material before application of
force. In engineering stress terms, the range for σe may be
from as little as 700 MPa, 900 MPa or 1100 MPa to as much as 1350 MPa,
1450 MPa or 1600 MPa, or within any range encompassed by the foregoing
values, to provide the increased benefits to the shape memory wire
discussed herein.

[0089]However, it is contemplated that force may be applied to the wire or
wire product using alternate stress loading regimes, such as via methods
other than a tensile test, that may be more appropriate to the geometry
of a particular wire product.

[0090]The temperature should be maintained below the martensitic
deformation temperature, Md, during the application of force to the
wire in the mechanical conditioning process. If the temperature exceeds
the martensitic deformation temperature, the bulk of the shape memory
wire material will not transform to martensite upon loading, because any
plastic deformation will occur in the austenite phase, and therefore the
localized phase transformation mechanism would not occur. The entirety of
the wire material will remain in a plastically deformed austenite state,
with the austenite or primary phase containing significant plasticity and
little retained martensite phase.

[0091]Referring to FIG. 6, once the desired stress or strain is applied to
the wire, the force is removed and the sample is allowed to freely
recover and, if the temperature is below the austenitic finish
temperature, or if further recovery of the bulk material is required,
heat Q is applied to drive the temperature, T to sufficiently greater
than the austenitic finish temperature (i.e., T>Af) for recovery
of the bulk material, leaving the material in a state wherein defects are
still isolated by a dislocation stabilized secondary phase as discussed
below.

[0092]The steps of applying a controlled engineering stress or strain and
subsequently removing the force to allow the sample to freely recover may
repeated as few as 1, 2 or 3 times or as many as 6, 8 or 10 times, for
example, to increase the amount of dislocation stabilized secondary phase
within the wire material, as discussed in detail below. Thus, after one
or more applications of load cycles and the attendant recovery of applied
strain εc the length of wire 10 is greater than its original
length Lo. More particularly, the length of wire 10 after load
conditioning is Lo+ps, where ps is the permanent set or isothermally
non-recoverable strain resulting from plastic, pseudoplastic and other
deformation mechanisms, as shown in FIG. 6. Some of this isothermally
non-recoverable strain can be recovered in the bulk material by slight
heating of the material as discussed above.

[0093]As discussed in more detail below in Section III, this isothermally
non-recoverable strain is indicative of the amount or volume of the wire
material that has been converted from the primary phase to the secondary
phase and, upon recovery of the wire material, remains stabilized in the
secondary material phase. These localized areas of secondary phase
material isolate defects and inhibit crack propagation in the primary
phase material.

[0094]For example, as calculated in Examples 3-7 below, this isothermally
non-recoverable strain may be calculated by measuring the difference in
wire length after load removal in a tensile test. Known tensile test
devices (including the test device used for the present Working Examples)
collect wire length data as the test is conducted. This data, not
presented herein, is used to generate the permanent set data presented in
the tables. This non-recoverable length, with the original length
subtracted therefrom, gives a positive value where isothermally
non-recoverable deformation has occurred (i.e., "permanent set"). This
difference can then be divided by the original length, the product of
which is a strain value representing the isothermally non-recoverable
strain. This amount arises from a residual volume of altered material
within the wire which has accommodated a given amount of strain not
recovered upon load removal. The observed isothermally non-recoverable
strain may be divided by the load plateau strain length, which is
associated with the forward transformation from parent austenite phase to
secondary, stress-induced, martensite phase, thereby providing a
quantitative indication of the volume fraction of altered material within
the wire.

[0095]This volume fraction, referred to as "max. volume martensite %" in
Tables 3-7 below is calculated using the following formula:

V m = INRS LPSL ##EQU00003##

wherein VM is the maximum volume fraction of secondary phase, INRS is
the isothermally non-recoverable strain and LPSL is the loading plateau
strain length.

[0096]The volume fraction sets an upper limit on the amount of wire
material that has been converted to the secondary phase from the primary
phase and remains stable at the given test temperature after load
removal. That is to say, the total volume of material represented by the
non-recoverable strain comprises secondary phase material, and may also
comprise other non-primary phase material arising from plastically
deformed primary phase material or other deformation phenomena.

[0097]It is counter-intuitive that the application of stress to a wire
product at a level sufficient to initiate plastic deformation according
to the present mechanical conditioning process could be beneficial for
the use of that wire product in a medical device that utilizes the shape
memory or superelastic characteristic of the wire product. One of
ordinary skill in the art would consider a wire product made of a shape
memory material that has been subjected to a stress level sufficient to
induce any amount of plastic deformation to be compromised in its shape
memory or superelastic characteristic and therefore unsuitable for use in
a medical device in which this characteristic is desired.

II. DESCRIPTION OF MATERIAL PROPERTIES OF WIRE PRODUCTS MADE IN ACCORDANCE
WITH THE PRESENT MANUFACTURING PROCESS

[0098]Wire products made of shape memory materials or alloys that have
been subjected to the mechanical conditioning process of the present
disclosure exhibit several novel physical characteristics and/or novel
combinations of physical characteristics, including the following:

[0099]1. Isolation of Defects

[0100]Referring to FIGS. 7(a)-(b), shape memory wire 10 may have one or
more defects, such as internal defects 28 and/or external defects 30.
These defects may include extrinsic defects and/or intrinsic defects such
as inclusions or porosity as discussed above, for example.

[0101]These defects are isolated in localized fields or areas of secondary
phase material by subjecting the wire to mechanical conditioning, as
exemplified by the curve shown in FIG. 8. As discussed above, this may be
accomplished by applying an engineering stress (and concomitant
engineering strain) so that at least some parts of wire 10 experience
plastic deformation. In an exemplary embodiment of the present process,
however, nearly all of the strain may be recovered upon unloading (FIG.
8).

[0102]Referring now to FIGS. 9-10(c), mechanical conditioning results in
areas of dislocation stabilized B19', R, and/or martensite, shown as
secondary phase areas 26 in FIG. 10, forming proximate defects 28 in wire
10. The formation of the secondary phase areas around and/or adjacent
defects 28 during mechanical conditioning helps to retard fatigue crack
growth in subsequent cyclic loading in a direction emanating from defects
28, as it is known that cracks propagate more slowly in B19', R, and/or
martensite than in austenite. However, the bulk of wire 10, where defects
are not present, reverts back to the austenite phase after mechanical
conditioning, such that the overall wire still exhibits its shape memory
or superelastic characteristic while at the same time having an enhanced
degree of fatigue strength due to the isolation of defects within the
secondary phase material.

[0103]Referring still to FIGS. 9-10(c), wires 10, 10' and 10'' are shown
after mechanical conditioning. As a result of mechanical conditioning in
accordance with the present process, areas of dislocation stabilized
secondary material phase 26 formed proximate material defects stabilize
the defects. That is to say, while the bulk of the wire material reverts
back to the primary phase from the secondary phase, localized areas of
secondary phase material remain formed proximate material defects. This
stabilization of the secondary phase areas 26 is advantageous in that it
helps to retard fatigue crack growth in subsequent cyclic loading, for
example, as a crack generally propagates more slowly in the secondary
(i.e., martensite) phase than in the primary (i.e., austenite) phase.

[0104]Stabilization of secondary phase areas 26 is, at least in part, due
to plastic deformation 27 comprising dislocations and/or dislocation
networks. This plastic deformation acts to stabilize secondary phase 26
after removal of the conditioning mechanical conditioning stress or
strain (when primary phase returns to the bulk of the wire material) and
during subsequent service. The defect-free portions of the wire material
may have less plastic deformation, or may have substantially no plastic
deformation. Therefore, this defect-free material will revert back to the
primary phase more readily and completely than the localized secondary
phase areas near the defects, which have experienced plastic deformation.

[0105]As shown in FIGS. 10(a)-(c), the shape, size and/or spatial
configuration of secondary phase 26 varies depending upon the
characteristics of the defect proximate the localized secondary phase
field. In general, the secondary phase field will form around the highest
stress areas of the defect, and may not form at lower stress areas. This
is because plastic deformation occurs most readily at the site of stress
concentrators during the mechanical conditioning process; the dislocation
stabilized secondary phase areas, which include some plastic deformation
27, will form at these stress concentration points even though the
primary phase portions of the wire are still within a relatively elastic
or pseudoelastic (where pseudoelastic is defined as elasticity associated
with primary to secondary phase transformation) deformation range.
Primary phase material remains present between any pair of defects that
are sufficiently far apart, such that their isolation fields to not
overlap.

[0106]For example, wire 10 shown in FIG. 10(a) has a secondary phase area
26 extending around substantially the entirety of defect 28. The geometry
of defect 28, as well as the direction application of force F, determines
the overall shape of secondary phase area 26.

[0107]As shown in FIG. 10(b), wire 10' has defects 28' with stabilized
secondary phase areas 26' at the highest stress concentration areas
created by the application of force F. Plastic deformation 27 also occurs
within secondary phase areas 26' as discussed above. Some of secondary
phase areas 26' are adjacent one another and have overlapping boundaries,
so that a defect 28' and another nearby defect 28' will be influence one
another.

[0108]Similarly, wire 10'' shown in FIG. 10(c) has multiple defects 28''
with stress fields 26'' and plastic deformation 27. Again, the stress
fields 28'' form at the highest stress concentration points, which are a
function of the geometry of defects 28'' and the direction of application
of force F (shown as a longitudinal force along the axis of wire 10'') as
well as the temperature of the material during force application.

[0109]In this manner, shape memory material wire subjected to the present
mechanical conditioning process exhibits an enhanced fatigue life and
fatigue strain threshold. Moreover, a shape memory wire made in
accordance with the present process retains overall material properties
consistent with wire in the austenitic phase, while exhibiting inhibition
of crack propagation at defect sites consistent with the martensitic
phase.

[0110]2. Increased High-Cycle Fatigue Resistance

[0111]As a result of the isolation of defects and/or defect boundaries
(i.e. the sites along the defect/primary phase boundary most susceptible
to stress concentration and crack propagation) in a secondary phase area
or field, mechanical conditioning increases the fatigue life and
fatigue-strain threshold of the shape memory wire. As discussed in
Section III, wire conditioned in accordance with an embodiment of the
present disclosure exhibited a gain in the fatigue strain limit at 100
million (108) cycles of greater than 25% (FIG. 11). Also, as shown
in FIG. 14, conditioned wire demonstrated an upward strain shift of
greater than 20% at a 10 million (107) cycle life (i.e., 1.1%
engineering strain versus 0.9% engineering strain). Further, eight
samples of this conditioned material survived more than 109 cycles
and were still running at the time conclusion of Example I discussed
below.

[0112]3. Increased Damage Tolerance and Low-Cycle Fatigue Resistance

[0113]As discussed in Section III at Example 2, wire conditioning in
accordance with an embodiment of the present disclosure demonstrated
increased tolerance of damage to the wire. Three specimens with focused
ion beam (FIB)-milled sharp defects were tested at an alternating strain
of 1% in the conditioned and non-conditioned states. As shown in FIG. 15,
Conditioned samples demonstrated a 50% increase in damage tolerance
compared with the non-conditioned samples.

[0116]This recoverable strain renders wire made in accordance with the
present disclosure particularly suitable for certain medical device
applications. As mentioned above, this is a counter-intuitive result.
Typically, wire made of a shape memory or superelastic material which has
been subjected to forces sufficient to cause any plastic deformation in
the wire material would be considered "damaged", and therefore unsuitable
for use in any medical device application. The surprising result of the
present process is that, under proper mechanical conditioning parameters
as discussed herein, wire subjected to such forces is actually superior
for medical device applications.

III. EXAMPLES

[0117]The following non-limiting Examples illustrate various features and
characteristics of the present invention, which are not to be construed
as limited thereto.

[0118]The examples offer analysis of the effect of mechanical conditioning
in fine (such as less than 250 μm diameter) Nitinol wire, and
particularly of the effect of stress riser or defect isolation.

Example 1

Fatigue Resistant Nitinol Intermetallic Wire

[0119]Nanocrystalline, nominally Ti-56 wt. % Ni Nitinol wire ("NiTi wire")
was manufactured to create a superelastic, precipitate free wire with a
median grain size of 50 nm. An exemplary process for creating such a wire
is described in U.S. patent application Ser. No. 12/563,062, entitled
FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed
Sep. 18, 2009, the disclosure of which is hereby expressly incorporated
by reference herein in its entirety.

[0120]The resulting wire was drawn through diamond drawing dies beginning
at a diameter of 0.230 mm and ending at a diameter of 0.177 mm to yield a
retained cold work level of about 40% cold work. The wire was then
continuously annealed at a temperature of 773K-873K for less than 60
seconds, to yield a 50 nm grain size as verified by TEM electron
microscopy scanning. Specifically, field emission scanning electron
microscopy or transmission electron microscopy (TEM) is used to gather an
image containing, for example, several hundred crystals or grains
exhibiting strong grain boundary contrast. Next, the image is converted
to a binary format suitable for particle measurement. Resolvable grains
are modeled with ellipsoids and subsequently measured digitally yielding
statistics regarding the crystal or grain size, such as the average size,
maximum size, and minimum size. The resulting average crystal size is
taken to be the average crystal size for the material from which the
sample was taken. Grain size verification is discussed in detail in U.S.
patent application Ser. No. 12/563,062, entitled FATIGUE DAMAGE RESISTANT
WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009, incorporated
by reference above.

[0121]After annealing, the wire of the present Example was subjected to
cyclic tensile testing and was determined to exhibit pseudoelasticity out
to greater than 10% engineering strain. A first wire sample was preserved
at this point in the process.

[0122]The remainder of the wire was then subjected to mechanical
conditioning, as described in detail above, by loading the wire to about
12% axial engineering strain (i.e., 0.12 units), completely releasing the
load, reloading to about 12% engineering strain, and once again
completely releasing the load. A second wire sample was preserved after
this point in the process.

[0123]The first and second wire samples were subjected to rotary beam
fatigue testing. Referring now to FIG. 11, the first, non-mechanically
conditioned sample generated data curve 100 exhibiting a 100 M cycle
engineering strain limit of about 0.85% at N=10 data points, shown by
right most data point 100' of the curve 100. The second, mechanically
conditioned sample generated data curve 102 exhibiting a 100 M cycle
engineering strain limit of about 1.1% at N=10 data points, exemplifying
a greater than 25% gain in the fatigue strain limit at 100 M cycles,
shown by right most data point 102' of curve 102.

[0124]In this example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were investigated. In thin wires, where plane stress
dominates, it is expected that sufficient loading will result in phase
transformation near the largest or shape-conducive crack-like defects,
such as constituent inclusion particles, before conversion of the bulk,
defect-free material.

[0125]1. Experimental Technique

[0126]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, of about 11.5%.
Conditioning was applied by approaching the martensitic yield point at
295 K using strain-rate-controlled loading in order induce some
dislocation locking of stress-transformed material in the vicinity of
stress concentrators. Referring now to FIG. 8, the conditioning cycle
comprises a strain-controlled ramp to a stress level of 1240 MPa
engineering stress, resulting in an engineering strain of about 11.5%,
followed by a 3 second hold, and finishing with a strain-controlled ramp
to zero load.

[0127]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, AS, of 243 K, having Ti-56 wt. % Ni
was repetitively drawn and annealed from a diameter of 2 mm to a diameter
of 177 μm in accordance with the process described above. At this
stage, wires were continuously annealed at 770 to 800 K. Final cold
working was completed using diamond dies to draw round wire with a
diameter of 150 μm prior to continuous, reel-to-reel annealing at 750
to 780 K under constant engineering stress for less than 60 seconds to
effect linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 280 K and an approximately 120 nm thick,
dark brown oxide layer similar to that disclosed in an article by the
present inventor entitled "Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire" and
published in the Journal of Materials Engineering and Performance,
February 2008, ISSN 1544-1024, pgs. 1-6, the entire disclosure of which
is hereby expressly incorporated by reference herein.

[0128]Focused ion beam (FIB)-milled sharp defects 202 ("FSD") were then
milled into material 200 of each sample in order to act as preferential
sites of incipient fatigue crack formation and to facilitate user-defined
damage localization and monitoring. An FEI dual-beam (Nova 200 NanoLab)
focused ion beam (FIB) with in situ scanning electron microscopy (SEM)
was used to simultaneously monitor samples by SEM during the FIB milling
process. A 30 keV Ga+ ion beam was used to precisely mill transverse
defects into wire specimens at a 0.50 nA beam current. Defects 202 were
of consistent dimension measuring 10 μm transverse length by 3 μm
radial depth by 0.5 μm axial surface width, an example of which is
shown in FIGS. 12(a)-(e). Cue lines 204 (FIG. 12(a)) were milled into the
oxide surface at a depth of about 50 nm on either side of each sharp
defect in order to enhance optical detection for accurate placement in
fatigue test gages after removal from the SEM chamber. As shown in FIGS.
12(a)-(e), the cue lines were of sufficient depth to create a visually
detectible gradient associated with the reduced oxide thickness, while
shallow enough to minimize undesirable mechanical impact.

[0129]Electron microscopy of fracture surfaces was carried out using a
Hitachi 54800 field emission SEM (FE-SEM) operated at 10 to 20 kV.
Transmission electron microscopy samples were extracted and prepared
using the FIB/SEM dual beam equipment previously mentioned with an in
situ sample manipulator for thin foil removal and transport to TEM grids.
Additional details regarding this method can be found in the article by
the present inventor, which is incorporated by reference herein above,
namely the article entitled "Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire" and
published in the Journal of Materials Engineering and Performance,
February 2008, ISSN 1544-1024, pgs. 1-6. TEM imaging and diffraction
experiments were carried out on a 200 kV machine equipped with a
LaB6 emitter (Tecnai 20, FEI Company, Oregon).

[0130]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip. Elevated temperature testing was completed on an
equivalent tensile bench fitted with an environmental chamber capable of
maintaining a temperature of 310±0.5 K.

[0131]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency. Data
have recently been presented by Robertson and Ritchie, in an article
entitled "In vitro fatiguecrack growth and fracture toughness behavior of
thin-walled superelastic Nitinol tube for endovascular stents: A basis
for defining the effect of crack-like defects", published in Biomaterials
28, 2007, pgs. 700-709, suggesting that high rate testing may well
estimate in vivo fatigue failure lifetimes.

[0132]As shown in FIGS. 13(a)-13(c), specimens from each group,
non-conditioned (NC) and conditioned (C), were tested at alternating
engineering strain (1/2 peak-to-peak amplitude) levels ranging from 0.8
to 1.6% to a maximum of about 109 cycles or 200 days test time.
Further, specimens with FSD 202 were tested at 1% engineering strain
before and after conditioning. The FSD zone was located at the apex of
the fatigue bend by optical positioning using the cue marks 204 as
guides.

[0133]2. Results

[0134]The resulting tensile data is shown graphically in FIGS. 13(a)-(c).
As noted above, the conditioning cycle comprised a strain-controlled ramp
to a stress level of 1240 MPa engineering stress followed by a 3 second
hold, finishing with a strain-controlled ramp to zero load. This
conditioning cycle generated data curve 220, shown in FIGS. 13(a)-(c).
The total engineering strain departure for this cycle, measured by
crosshead extension, was 11.5%. Conditioning initially resulted in 0.3%
residual strain comprising both plastic and pseudo-plastic strain
contributions.

[0135]The martensite to austenite reversion plateau stress associated with
unloading was significantly reduced during unloading from the
conditioning cycle, but was observed to elevate in subsequent testing to
8% engineering strain. Some of this effect can be accounted for by strain
rate differences: the conditioning cycle was run at a significantly
higher strain rate than the 8% test cycles. High strain rates can cause
heating during loading and cooling during unloading resulting in
increasing stress hysteresis. Further testing of a C sample at body
temperature (310 K) generated data curve 226 depicted on FIGS. 13(a)-(c)
by circular marks 226', showing elevation of the unloading plateau stress
to levels greater than an NC sample at 295 K, shown as data curve 222
with square-shaped marks 222. This result was consistent with known test
temperature-plateau stress relationships.

[0136]The conditioned sample at 295K, shown as curve 224 on FIGS.
13(a)-(c) with triangle-shaped marks 224', exhibited a downward shift in
the unloading plateau stress. This can be attributed to plastic
deformation, some of which may be directly beneficial to resistance
against subsequent fatigue crack growth. The lack of significant shift in
the strain length of the plateaus indicates that plastic deformation to
the overall microstructure was minimal during overload conditioning.

[0137]FIG. 14 illustrates the observed differences in fatigue performance
for conditioned (C) wire specimens, shown as data curve 230 with
triangular-shaped marks 230', versus non-conditioned (NC) wire specimens,
shown as data curve 232 with triangular-shaped marks 232'. The
conditioning resulted in an upward strain shift of greater than 20% at
the 107 cycle life (i.e., 1.1% engineering strain versus 0.9%
engineering strain). Eight samples of the conditioned material survived
more than 109 cycles and were still running at the time conclusion
of the experiment.

[0138]As shown in FIG. 15, three FSD specimens in each of the NC and C
states were tested at an alternating strain of 1%. In this case, the
FSD-C group generated data bar graph 242, showing an average of 21,228
cycles to failure with a margin of error indicated at the top of the bar.
The FSD-NC group generated data bar graph 240, showing an average of
14,196 cycles to failure with a margin of error indicated at the top of
the bar. Thus, the conditioned wire samples outperformed the
non-conditioned samples by 50%. All FSD samples failed considerably
before the non-FSD samples; this is attributable to the geometry of the
FIB-milled defects, which were purposefully milled larger and sharper
than the 2-6 μm inclusion particles typically found at fatigue failure
sites in this grade of Nitinol wire in order to direct site-specific,
locatable failure for study.

[0139]A microstructurally distinct region resulting from the mechanical
conditioning was found within an approximately 500 nm radius of the
approximately 10 nm width FSD crack root. FIG. 16 shows the results of
TEM work performed to help elucidate mechanisms giving rise to mechanical
property changes associated with the mechanical conditioning. The
selected area diffraction patterns 250, 252 in FIG. 16, outside of and
within the distinct FSD concentration zone respectively, reveal
significant differences in contributing bright field contrast signal. A
typical polycrystalline, B2 pattern 250 was observed at approximately 1
μm from the crack tip, while the selected area diffraction pattern
adjacent to the root, shown as 252, revealed what appears to be
superimposed diffuse rings, B2 polycrystalline reflections, as well as
some evidence of 1/2(110) reflections associated with the B19'
martensitic phase. Also evident is a significant increase in dislocation
density and associated contrast.

[0140]The diffuse (110) rings observed within the FSD zone, shown in the
left and upper-right insets of FIG. 16, may be related to partial
amorphization and/or due to (110) reflection splitting and the presence
of 1/2(110) reflections associated with a mixed B2-B19' structure.

[0142]The narrowest ductile striations were observed in conditioned
samples near the FSD incipient crack front. High resolution SEM (HR-SEM)
analysis of fatigue failure sites in NC and C specimens was completed and
the stress intensity was estimated based on the probable crack front
location at the examined site using assumptions of a semi-elliptical
crack in an infinite cylinder. Referring to FIG. 17, a crack growth rate
plot as a function of the estimated stress intensity factor (not taking
into account crack closure effects) is shown, with square-shaped marks
260 indicating data on non-conditioned material and triangle-shaped marks
262 indicating conditioned material. The difference between the two data
sets may suggest martensitic growth rate inhibition.

[0143]In this example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
greater than 8% recoverable engineering strain was observed with zero
residual strain and good plateau stresses at body temperature after
mechanical conditioning. Further, an increase in the strain fatigue limit
of greater than 20% at 107 cycles is observed in conditioned versus
non-conditioned wire with an observed increase in low cycle life of 50%.
The tensile overload conditioning treatment also resulted in a
mixed-phase microstructure in the vicinity of stress concentrators that
comprises increased dislocation density and possible plasticity-induced
or roughness-induced crack closure.

Introduction to Examples 3-7

[0144]For examples 3-7, various wire materials were tested in a similar
manner for comparison to one another. FIGS. 18(a)-(d) show results for
wire materials from each of Examples 3-7, with each Figure representing a
different strain condition (as indicated on each respective Figure).
Table 1, below, indexes the Example materials:

[0145]FIGS. 19(a)-(e) also show results for wire materials from each of
Examples 3-7, with each Figure representing a stress-strain curve for a
corresponding Example. Table 2, below, indexes the mechanical
conditioning regimes applied to each wire product shown in FIGS.
19(a)-(e). The "regime" number corresponds to a given curve on each
figure, as indicated by the corresponding number on the legend at the
right side of each of FIGS. 19(a)-(e), where, the digit preceding the
decimal point in X.X refers to the Example (e.g. 1, 2, 3, 4, and 5
correspond to Examples 3, 4, 5, 6, and 7 respectively), and the digit
following the decimal point corresponds to regimes 1, 3, 4, 6, and 7
given below in Table 2.

[0147]In this example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were further investigated over a broader range of loads as
compared to Example 2.

[0148]1. Experimental Technique

[0149]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, ranging from about 8%
to 12.5% Conditioning was applied by approaching the martensitic yield
point at 295 K using strain-rate-controlled loading in order induce some
dislocation locking of stress-transformed material in the vicinity of
stress concentrators. Referring now to FIG. 19(a), the conditioning cycle
comprises a strain-controlled ramp to five stress levels of 700, 1100,
1240, 1400, and 1500 MPa engineering stress, resulting in an engineering
strain of about 8.3%, 9.8%, 10.3%, 11.1% and 12.2% respectively, followed
by a 3 second hold, finishing with a strain-controlled ramp to zero load.

[0150]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, As, of 243 K, having Ti-56 wt. %
Ni was repetitively drawn and annealed from a diameter of 2 mm to a
diameter of 201 μm in accordance with the process described above. At
this stage, wires were continuously annealed at 950 to 1000 K. Final cold
working was completed using diamond dies to draw round wire with a
diameter of 151 μm prior to continuous, reel-to-reel annealing at 750
to 780 K under constant engineering stress for 40 to 80 seconds to effect
linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 280 K and an approximately 120 nm thick,
dark brown oxide layer similar to that disclosed in an article by the
present inventor entitled "Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire" and
published in the Journal of Materials Engineering and Performance,
February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by
reference above.

[0151]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip.

[0152]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency.

[0153]As shown in FIGS. 18(a)-(d), three specimens from each group at each
conditioning cycle, ranging from 0 MPa which indicates non-conditioned
wire to 1500 MPa which indicates the maximum conditioning load used, were
tested at alternating engineering strain (1/2 peak-to-peak amplitude)
levels ranging from 0.80 to 1.25% strain to a maximum of about 106
cycles. The total samples tested for each conditioning load regime for
each sample was 12 resulting in a total of 72 fatigue samples tested for
this portion of the study. Samples which did not fracture after 106
cycles were stopped and recorded.

[0154]2. Results

[0155]The resulting tensile data is shown graphically in FIGS. 18(a)-(d)
as line 1 on each curve. As noted above, the conditioning cycle comprised
a strain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or
1500 MPa engineering stress, as indicated along the horizontal axis of
the plot, followed by a 3 second hold, finishing with a strain-controlled
ramp to zero load.

[0157]FIGS. 18 (a) to (d) illustrate the observed differences in fatigue
performance for non-conditioned (e.g. 0 load level on x-axis) and
conditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on the
x-axis) The conditioning resulted in an upward cycle life shift of at
least 55% and 3000% at the 1.25% and 0.95% alternating strain test levels
respectively at a conditioning load level of 1240 MPa. An overall upward
trend in lifetime for a given test strain level was observed for
increasing conditioning load through 1500 MPa. Most samples of the
material conditioned at greater than 1240 MPa survived more than 106
cycles and were still running at the time of conclusion of the experiment
for test strain levels below 0.95%.

[0158]FIG. 19(a) illustrates the observed tensile behavior during load
conditioning of each sample. In each case, an upper bound of the maximum
volume of retained martensite was calculated as described above based on
the ratio of isothermally non-recoverable strain to the strain length of
the load plateau. FIG. 20 illustrates the positive correlation between
isothermally non-recoverable strain and conditioning load.
Non-recoverable strain was less than 0.17% for all samples conditioned
below 1400 MPa resulting in a max. volume of retained martensite estimate
of 3.7% for the same samples loaded below 1400 MPa.

[0159]In this Example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
less than 3.7% of the matrix was left in the martensite phase after load
removal for conditioning below 1400 MPa with a concomitant maximum
isothermally non-recoverable strain of 0.17%. Further, an increase in the
strain fatigue life of greater than 3000% at 106 cycles is observed
in wire conditioned at 1240 MPa versus non-conditioned wire while
maintaining good elastic properties suitable for said medical device
applications.

[0160]In this Example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were further investigated over a broader range of loads as
compared to Example 2 using a high strength chromium doped tertiary
Nitinol compound.

[0161]1. Experimental Technique

[0162]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, ranging from about
7.7% to 13.1% Conditioning was applied by approaching the martensitic
yield point at 295 K using strain-rate-controlled loading in order induce
some dislocation locking of stress-transformed material in the vicinity
of stress concentrators. Referring now to FIG. 19(b), the conditioning
cycle comprises a strain-controlled ramp to five stress levels of 0, 700,
1100, 1240, 1400, and 1500 MPa engineering stress, resulting in an
engineering strain of about 0%, 7.7%, 9.6%, 10.2%, 11.3%, and 13.1%
respectively, followed by a 3 second hold, finishing with a
strain-controlled ramp to zero load.

[0163]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, As, of about 235 K, having
Ti-55.8 wt. % Ni-0.25 wt % Cr was repetitively drawn and annealed from a
diameter of 2 mm to a diameter of 361 nm in accordance with the process
described above. At this stage, wires were continuously annealed at 950
to 1000 K. Final cold working was completed using diamond dies to draw
round wire with a diameter of 269 nm prior to continuous, reel-to-reel
annealing at 750 to 780 K under constant engineering stress for 40 to 80
seconds to effect linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 283 K and an approximately 120 nm thick,
dark brown oxide layer similar to that disclosed in an article by the
present inventor entitled "Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire" and
published in the Journal of Materials Engineering and Performance,
February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by
reference above.

[0164]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip.

[0165]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency.

[0166]As shown in FIGS. 18 (a) to (d), three specimens from each group at
each conditioning cycle, ranging from 0 MPa which indicates
non-conditioned wire to 1500 MPa which indicates the maximum conditioning
load used, were tested at alternating engineering strain (1/2
peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a
maximum of about 106 cycles. The total samples tested for each
conditioning load regime for each sample was 12 resulting in a total of
72 fatigue samples tested for this portion of the study. Samples which
did not fracture after 106 cycles were stopped and recorded.

[0169]FIGS. 18 (a)-(d) illustrate the observed differences in fatigue
performance for non-conditioned (e.g. 0 load level on x-axis) and
conditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on the
x-axis) The conditioning resulted in an upward cycle life shift of at
least 46% and 2500% at the 1.25% and 0.95% alternating strain test levels
respectively at a conditioning load level of 1240 MPa. An overall upward
trend in lifetime for a given test strain level was observed for
increasing conditioning load through 1500 MPa. Most samples of the
material conditioned at greater than 1240 MPa survived more than 106
cycles and were still running at the time of conclusion of the experiment
for test strain levels below 0.95%.

[0170]FIG. 19(b) illustrates the observed tensile behavior during load
conditioning of each sample. In each case, an upper bound of the maximum
volume of retained martensite was calculated as described above based on
the ratio of isothermally non-recoverable strain to the strain length of
the load plateau. FIG. 20 illustrates the positive correlation between
isothermally non-recoverable strain and conditioning load.
Non-recoverable strain was less than 0.35% for all samples conditioned
below 1400 MPa resulting in a max volume of retained martensite estimate
of 5.9% for the same samples loaded below 1400 MPa.

[0171]In this Example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
less than 5.9% of the matrix was left in the martensite phase after load
removal for conditioning below 1400 MPa with a concomitant maximum
isothermally non-recoverable strain of 0.35%. Further, an increase in the
strain fatigue life of greater than 2500% at 106 cycles is observed
in wire conditioned at 1240 MPa versus non-conditioned wire while
maintaining good elastic properties suitable for said medical device
applications.

[0172]In this Example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were further investigated over a broader range of loads as
compared to Example 2 using a Nitinol with warmer transformation
temperatures as compared to Examples 2 and 3.

[0173]1. Experimental Technique

[0174]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, ranging from about
8.3% to 12.8% Conditioning was applied by approaching the martensitic
yield point at 295 K using strain-rate-controlled loading in order induce
some dislocation locking of stress-transformed material in the vicinity
of stress concentrators. Referring now to FIG. 19 (c), the conditioning
cycle comprises a strain-controlled ramp to five stress levels of 0, 700,
1100, 1240, and 1400 MPa engineering stress, resulting in an engineering
strain of about 0%, 8.3%, 10%, 10.8%, and 12.8% respectively, followed by
a 3 second hold, finishing with a strain-controlled ramp to zero load.

[0175]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, As, of about 255 K, having
Ti-55.8 wt. % Ni was repetitively drawn and annealed from a diameter of 2
mm to a diameter of 380 μm in accordance with the process described
above. At this stage, wires were continuously annealed at 950 to 1000 K.
Final cold working was completed using diamond dies to draw round wire
with a diameter of 302 μm prior to continuous, reel-to-reel annealing
at 750 to 780 K under constant engineering stress for 40 to 80 seconds to
effect linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 298 K and an approximately 120 nm thick,
dark brown oxide layer similar to that disclosed in an article by the
present inventor entitled "Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire" and
published in the Journal of Materials Engineering and Performance,
February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated by
reference above.

[0176]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip.

[0177]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency.

[0178]As shown in FIGS. 18 (a) to (d), three specimens from each group at
each conditioning cycle, ranging from 0 MPa which indicates
non-conditioned wire to 1400 MPa which indicates the maximum conditioning
load used, were tested at alternating engineering strain (1/2
peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a
maximum of about 106 cycles. The total samples tested for each
conditioning load regime for each sample was 12 resulting in a total of
60 fatigue samples tested for this portion of the study. Samples which
did not fracture after 106 cycles were stopped and recorded.

[0179]2. Results

[0180]The resulting tensile data is shown graphically in FIG. 19 (c). As
noted above, the conditioning cycle comprised a strain-controlled ramp to
a stress level of 0, 700, 1100, 1240, or 1400 MPa engineering stress, as
indicated along the horizontal axis of the plot, followed by a 3 second
hold, finishing with a strain-controlled ramp to zero load. This
conditioning cycle generated curves 3.1, 3.3, 3.4, and 3.6 shown in FIG.
19(c). The total engineering strain departure for respective cycles,
measured by crosshead extension, was about 0%, 8.3%, 10%, 10.8%, and
12.8% for increasing load levels respectively. Conditioning resulted in
residual strains (i.e. isothermally non-recoverable strains) of
respectively 0%, 0.03%, 0.13%, 1.1%, and 7.31% respectively.

[0181]FIGS. 18 (a) to (d) illustrate the observed differences in fatigue
performance for non-conditioned (e.g. 0 load level on x-axis) and
conditioned wire specimens (700, 1100, 1240, and 1400 MPa on the x-axis)
The conditioning resulted in an upward cycle life shift of at least 116%
and 2400% at the 1.25% and 0.95% alternating strain test levels
respectively at a conditioning load level of 1240 MPa. An overall upward
trend in lifetime for a given test strain level was observed for
increasing conditioning load through 1400 MPa. Most samples of the
material conditioned at greater than 1240 MPa survived more than 106
cycles and were still running at the time of conclusion of the experiment
for test strain levels below 0.95%.

[0182]FIG. 19(c) illustrates the observed tensile behavior during load
conditioning of each sample. In each case, an upper bound of the maximum
volume of retained martensite was calculated as described above based on
the ratio of isothermally non-recoverable strain to the strain length of
the load plateau. FIG. 20 illustrates the positive correlation between
isothermally non-recoverable strain and conditioning load.
Non-recoverable strain was less than about 1% for all samples conditioned
below 1240 MPa resulting in a max. volume of retained martensite estimate
of about 17% for the same samples loaded below 1240 MPa.

[0183]In this Example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
less than about 17% of the matrix was left in the martensite phase after
load removal for conditioning below 1240 MPa with a concomitant maximum
isothermally non-recoverable strain of about 1%. Further, an increase in
the strain fatigue life of greater than 2400% at 106 cycles is
observed in wire conditioned at 1240 MPa versus non-conditioned wire
while maintaining good elastic properties suitable for said medical
device applications.

[0184]In this Example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were further investigated over a broader range of loads as
compared to Example 2 and in a finer diameter using a Nitinol with an
etched surface finish comprising a substantially oxide-free surface.

[0185]1. Experimental Technique

[0186]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, ranging from about 7.8
to 11.9% Conditioning was applied by approaching the martensitic yield
point at 295 K using strain-rate-controlled loading in order induce some
dislocation locking of stress-transformed material in the vicinity of
stress concentrators. Referring now to FIG. 19 (d), the conditioning
cycle comprises a strain-controlled ramp to five stress levels of 0, 700,
1100, 1240, 1400 and 1500 MPa engineering stress, resulting in an
engineering strain of about 0%, 7.8%, 9.5%, 10%, 10.8%, and 11.9%
respectively, followed by a 3 second hold, finishing with a
strain-controlled ramp to zero load.

[0187]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, As, of about 246 K, having Ti-56
wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a
diameter of 102 nm in accordance with the process described above. At
this stage, wires were continuously annealed at 950 to 1000 K. Final cold
working was completed using diamond dies to draw round wire with a
diameter of 76 nm prior to continuous, reel-to-reel annealing at 750 to
780 K under constant engineering stress for 40 to 80 seconds to effect
linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 288 K and an etched, substantially
oxide-free surface finish.

[0188]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip.

[0189]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency.

[0190]As shown in FIGS. 18 (a) to (d), three specimens from each group at
each conditioning cycle, ranging from 0 MPa which indicates
non-conditioned wire to 1500 MPa which indicates the maximum conditioning
load used, were tested at alternating engineering strain (1/2
peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a
maximum of about 106 cycles. The total samples tested for each
conditioning load regime for each sample was 12 resulting in a total of
72 fatigue samples tested for this portion of the study. Samples which
did not fracture after 106 cycles were stopped and recorded.

[0193]FIGS. 18 (a) to (d) illustrate the observed differences in fatigue
performance for non-conditioned (e.g. 0 load level on x-axis) and
conditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on the
x-axis) The conditioning resulted in an upward cycle life shift of at
least 7.2% and 2600% at the 1.25% and 0.95% alternating strain test
levels respectively at a conditioning load level of 1240 MPa. An overall
upward trend in lifetime for a given test strain level was observed for
increasing conditioning load through 1500 MPa. Most samples of the
material conditioned at greater than 1240 MPa survived more than 106
cycles and were still running at the time of conclusion of the experiment
for test strain levels below 0.95%.

[0194]FIG. 19(d) illustrates the observed tensile behavior during load
conditioning of each sample. In each case, an upper bound of the maximum
volume of retained martensite was calculated as described above based on
the ratio of isothermally non-recoverable strain to the strain length of
the load plateau. FIG. 20 illustrates the positive correlation between
isothermally non-recoverable strain and conditioning load.
Non-recoverable strain was less than about 0.29% for all samples
conditioned below 1240 MPa resulting in a max. volume of retained
martensite estimate of about 4.7% for the same samples loaded below 1400
MPa.

[0195]In this Example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
less than about 4.7% of the matrix was left in the martensite phase after
load removal for conditioning below 1400 MPa with a concomitant maximum
isothermally non-recoverable strain of about 0.29%. Further, an increase
in the strain fatigue life of greater than 2600% at 106 cycles is
observed in wire conditioned at 1240 MPa versus non-conditioned wire
while maintaining good elastic properties suitable for said medical
device applications.

[0196]In this Example, the effects of mechanical overload conditioning of
superelastic wire and the possibility of increased fatigue damage
resistance associated with near-defect, plasticity-locked phase
transformation were further investigated over a broader range of loads as
compared to Example 2 and in a larger diameter using a Nitinol with an
etched and mechanically polished surface finish comprising a
substantially oxide-free surface.

[0197]1. Experimental Technique

[0198]Samples for this Example were subjected to a total engineering
strain departure, measured by crosshead extension, ranging from about 7.5
to 13.1% Conditioning was applied by approaching the martensitic yield
point at 295 K using strain-rate-controlled loading in order induce some
dislocation locking of stress-transformed material in the vicinity of
stress concentrators. Referring now to FIG. 19 (d), the conditioning
cycle comprises a strain-controlled ramp to five stress levels of 0, 700,
1100, 1240, 1400 and 1500 MPa engineering stress, resulting in an
engineering strain of about 0%, 7.5%, 9.6%, 10.3%, 11.6%, and 13.1%
respectively, followed by a 3 second hold, finishing with a
strain-controlled ramp to zero load.

[0199]In order to prepare samples for this Example, Nitinol wire with an
ingot austenite start temperature, As, of about 249 K, having Ti-56
wt. % Ni was repetitively drawn and annealed from a diameter of 2 mm to a
diameter of 813 μm in accordance with the process described above. At
this stage, wires were continuously annealed at 950 to 1000 K. Final cold
working was completed using diamond dies to draw round wire with a
diameter of 638 μm prior to continuous, reel-to-reel annealing at 750
to 780 K under constant engineering stress for 60 to 150 seconds to
effect linear shape setting. The final wire comprised a
room-temperature-superelastic Nitinol wire with an active austenitic
finish temperature, Af, of 291 K and an etched, substantially
oxide-free and mechanically polished surface finish.

[0200]Cyclic and monotonic uniaxial tensile properties were measured at an
ambient temperature of 295 K at a strain rate of 10-3 s-1 using
an Instron Model 5565 Tensile Test Bench equipped with pneumatic, smooth
face grips. Six hundred grit emery-cloth was used to reduce grip-specimen
interface slip.

[0201]Fatigue behavior was characterized using rotary beam fatigue test
equipment manufactured by Positool, Inc., at a test rate of 60 s-1
in ambient 298 K air. The test rate was chosen at a rate significantly
higher than physiological loading frequencies to promote expediency.

[0202]As shown in FIGS. 18 (a) to (d), three specimens from each group at
each conditioning cycle, ranging from 0 MPa which indicates
non-conditioned wire to 1500 MPa which indicates the maximum conditioning
load used, were tested at alternating engineering strain (1/2
peak-to-peak amplitude) levels ranging from 0.80 to 1.25% strain to a
maximum of about 106 cycles. The total samples tested for each
conditioning load regime for each sample was 12 resulting in a total of
72 fatigue samples tested for this portion of the study. Samples which
did not fracture after 106 cycles were stopped and recorded.

[0205]FIGS. 18 (a) to (d) illustrate the observed differences in fatigue
performance for non-conditioned (e.g. 0 load level on x-axis) and
conditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on the
x-axis) The conditioning resulted in an upward cycle life shift of at
least 54% at the 0.95% alternating strain test levels respectively at a
conditioning load level of 1240 MPa. An overall upward trend in lifetime
for a given test strain level was observed for increasing conditioning
load through 1500 MPa. Most samples of the material conditioned at
greater than 1240 MPa survived more than 106 cycles and were still
running at the time of conclusion of the experiment for test strain
levels below 0.80%.

[0206]FIG. 19(e) illustrates the observed tensile behavior during load
conditioning of each sample. In each case, an upper bound of the maximum
volume of retained martensite was calculated as described above based on
the ratio of isothermally non-recoverable strain to the strain length of
the load plateau. FIG. 20 illustrates the positive correlation between
isothermally non-recoverable strain and conditioning load.
Non-recoverable strain was less than about 0.61% for all samples
conditioned below 1240 MPa resulting in a max. volume of retained
martensite estimate of about 11% for the same samples loaded below 1240
MPa.

[0207]In this Example, it has been demonstrated that mechanical
conditioning of superelastic NiTi wire results in improved fatigue
performance, while maintaining good mechanical properties. In addition,
less than about 11% of the matrix was left in the martensite phase after
load removal for conditioning below 1240 MPa with a concomitant maximum
isothermally non-recoverable strain of about 0.61%. Further, an increase
in the strain fatigue life of greater than 54% at 106 cycles is
observed in wire conditioned at 1240 MPa versus non-conditioned wire
while maintaining good elastic properties suitable for said medical
device applications.

[0208]Wires made in accordance with the present disclosure are susceptible
of a variety of applications including, but not limited to the
applications detailed below. Exemplary applications of wires in
accordance with the present disclosure are set forth below, and shown
generally in FIGS. 18(a)-19(b).

[0209]In some cases, a wire may have no remaining biased curvature, such
as in a percutaneous transluminal coronary angioplasty (PTCA), steerable,
and torque whip free guidewire application, or for a torque transmission
wire for coronary plaque removal, for example.

[0210]Wire products used for medical devices as discussed herein will
typically be subjected to mechanical conditioning in accordance with the
present disclosure prior to integration into a medical device. However,
it is contemplated that wire products may alternatively be installed
into, or at least partially configured as, a medical device prior to
subjecting the wire product to mechanical conditioning, followed by
conducting the mechanical conditioning on the wire product after same is
installed into, or at least partially configured as, a medical device, in
order to impart benefits as disclosed herein.

A. DFT® and Other Composite Wire Materials

[0211]Wires disclosed herein may be used for composite wire products, such
as shown in FIGS. 21(a)-(b). Composite wire 300 includes an outer shell
302 made of a first material, and a core 304 comprising at least one core
segment of a second material, and optionally, additional core segments of
third or more materials. Outer shell 302 may be made of a wire in
accordance with the present disclosure, and core 304 may have a variety
of desired properties, such as resistance, radiopacity, or any other
property.

[0212]Thus, composite wire 300 may confer the benefits of load-conditioned
and therefore fatigue damage resistant outer shell, such as fatigue
strength, low or zero permanent set, etc, as described above by applying
a suitable conditioning load to the wire product prior to installation
within a medical device and/or configuration as a medical device, while
also having other properties associated with the second material
comprising core 304. An exemplary composite wire product is DFT®,
available from Fort Wayne Metals Research Products Corp. of Fort Wayne,
Ind.

B. Shape Memory Devices

[0213]1. Wire-Based Stents

[0214]Referring to FIG. 22(a), a tissue scaffold or vessel stent device
370 is shown which is made from one or more wires 372 made in accordance
with the present process, which are braided, knitted, or otherwise formed
together to produce the generally cylindrical cross-sectional shape of
device 370.

[0215]Referring to and FIG. 22(b), a tissue scaffold or vessel stent
device 370' is shown which is made from one or more wires 372' made in
accordance with the present process, which are knitted together to form
the generally cylindrical cross-sectional shape of device 370'.

[0216]Upon release from the delivery catheter, stents move to some degree,
dependent on the relative vessel and device compliance, with the artery
due to fluctuations in blood pressure, arterial vessel smooth muscle
contraction and dilation, and due to general anatomical movement. Such
mechanical displacement results in cyclic straining of wires 372, 372'
comprising the structure of stent 370, 370' structure.

[0217]Non bioerodable tissue scaffolds or stents are generally implanted
permanently, and therefore should be able to withstand millions of
mechanical load cycles without losing structural integrity due to
mechanical fatigue.

[0218]Stents 370, 370', which are constructed from wires 372, 372' made in
accordance with the present process, possess a high degree of resistance
to fatigue damage and thus offer optimized performance as compared to
conventional stents made with wires having lower fatigue strength.

[0219]2. Blood Filters

[0220]Referring still to FIGS. 22(a)-(b), devices 370, 370' may also take
the form of a blood filter which is made from one or more wires 372, 372'
made in accordance with the present process, which are braided, knitted,
or otherwise formed together to produce the generally cylindrical
cross-sectional shape of devices 370, 370'. In this respect, many blood
filter devices may be similar to braided, knitted, or laser-cut stents,
and many interior vena cava (IVC) filters may be shaped as
umbrella-shaped devices. In use, the superelastic characteristic of the
device is utilized in that the device is inserted into a blood vessel via
catheter in a collapsed condition and is deployed by expansion into the
blood vessel, where the device captures and/or redirects larger blood
clots from critical anatomical organs or regions. In use, particularly in
permanent or non-retrievable devices, the device may be subjected to
repeated, movements such that high fatigue strength is desired.

[0221]While this invention has been described as having a preferred
design, the present invention can be further modified within the spirit
and scope of this disclosure. This application is therefore intended to
cover any variations, uses, or adaptations of the invention using its
general principles. Further, this application is intended to cover such
departures from the present disclosure as come within known or customary
practice in the art to which this invention pertains and which fall
within the limits of the appended claims.